The present invention relates to photographic elements, or more particularly to lithographic printing plates.
It is well known that most lithographic printing plates are formed by coating a diazo containing light sensitive composition onto an aluminum or treated aluminum substrate. Such a plate is exposed through a photographic mask to ultraviolet or actinic radiation thus causing the diazonium compound to decompose in the light struck image areas. The unexposed non-image areas still contain nondecomposed diazo and is removed by developing. One problem with such plates is that after exposure such plates still contain a certain amount of nondecomposed diazo in the image areas. The undecomposed diazo is sensitive to reaction to certain ingredients present in the developer, finisher or the fountain solution. Therefore, such a reaction might cause more hydrophilic areas on the surface of the coating and lead to a reduced ink receptivity in these areas. These problems manifest themselves in slow rollup, blinding, and poor contrast as demonstrated by many step wedge ghost steps.
It has been known in the art that non-light sensitive azo dyes can be produced by reacting diazo compounds with phenolic coupling agents under certain conditions. One approach to combine these features is described in European Patent Application No. 0024872. After the non-image active diazo portion is removed by development, the plate with only the image areas remaining is treated with a finisher to eliminate the active diazo in the image parts.
The disadvantage of this method is that only active diazo molecules on the surface of the image areas can be destroyed. Penetration of the diazo eliminating compounds substantially into the coating is not possible. Therefore, after a certain wear off of the coating during the press run, there are again active diazo molecules in the surface of image areas which are able to react with hydrophilic ingredients of the fountain solution or cleaning solutions used during machine down times.
U.S. Pat. Nos. 3,300,309 and 4,171,974 discuss the formation of light sensitive reaction products of diazonium salts and "coupling agents". However, such are not true coupling reactions. The reactions described in these disclosures are counter-ion replacement reactions whereby a water soluble diazonium salt ArN.sub.2.sup.- X.sup.+ has its X.sup.+ ion replaced with an aromatic hydroxy containing compound such that the new compound remains light sensitive but is rendered water insoluble. This is contrasted with true coupling reactions whereby the diazo is reacted to form a non-light sensitive azo compound of the formula: EQU Ar-N=N-Ar'
The present invention provides a negative working light sensitive coating containing a light sensitive diazonium salt in a stable, non-reacted, physical admixture with an aromatic carboxylic coupling agent in an acidic medium. The coating is preferably applied to a lithographic substrate and exposed to actinic radiation through a photographic mask. The coating is then developed with an alkaline developer such that the removal of the unexposed non-image areas is faster than the coupling reaction. This is necessary since the unexposed areas are almost exclusively diazo and will couple with the developer alkali. The developer effectively couples any reactive diazo remaining in the image areas.
The incorporation of couplers in the coating allows substantial elimination of active diazo in the bulk of the coating because the hydroxyl ions which promote the coupling reaction are very well able to penetrate to the bottom of the coating.
The composition of the present invention provides a stable physical admixture of a diazonium salt and certain aromatic carboxylic compounds having phenolic hydroxyl groups, in an acidic medium. The compositions provide improved light speed and development speed. Furthermore, in unexposed non-image areas the alkali developer has a faster dissolution speed than coupling speed. However, in almost totally exposed areas with a reduced solubility in the developer, the coupling reaction becomes predominant over the dissolution. In that way, additional solid steps are chemically formed by the reaction of diazo and coupler for a effective increase in practical light speed. Active diazo molecules remaining in image areas after exposure are deactivated by converting into an azo dye. Such a highly light sensitive coating is useful for laser imaging systems.